Illustrated Physics
physics, drawn
Concept

The Faraday cage — where it appears

An enclosure of conducting sheet or mesh that keeps electric fields out of its interior because charges on it rearrange to cancel them. A closed metal box shields completely against static fields; a cage of separate wires leaks the average field in proportion to the spacing between them.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A conductor in a field, with the surface charge solved for. Field lines approaching an isolated conducting cylinder. The surface charge was found by requiring the conductor to be an equipotential, and the lines then end on that charge, meeting the surface at right angles and leaving the interior empty.

The inside of a conductor, where the field is exactly nothing

Put a metal object in any electric field and the field inside it is zero. Not small — zero, by an argument that takes one sentence, and with consequences that reach from lightning to the most precise test of Coulomb's law ever made.

electromagnetism · Conductors A cage of wires in a uniform field. Equipotential lines round a cage of 12 parallel wires, each 2 per cent of the spacing in radius, joined so that they share one potential and carry no net charge, in a uniform field running left to right. Outside, the lines bunch and bend round the cage as they would round a solid cylinder. Inside, they do not vanish: they run straight across, carrying a uniform field 25.8 per cent as strong as the one outside (dashed, drawn at a fifth of the outside spacing because the potential differences inside are so much smaller). The wires' charges are computed so that every wire is at the same potential.

The field a ring of wires lets through

A cage made of wires is supposed to work almost as well as a solid box, because the field that leaks through the gaps dies away exponentially within a gap's width. The exponential is real and it is the wrong part of the story. It describes the ripple the separate wires make; underneath it the average field goes straight through, at a level that falls only in proportion to the spacing and depends on the wires' thickness through a logarithm. A cage of twelve thin wires lets a quarter of the field in.

electromagnetism · Potential

Named alongside it

The objects these essays reach for when they reach for this one.

Surface chargeBoundary conditionsConductorConductorsElectric fieldElectric potentialElectrostatic shieldingEquipotentialFourier seriesLaplace equationShielding

All concepts